Various types of rotating biological waste water contactors have been used to expose organisms to air to encourage their growth and thereby increase digestion of organic waste materials in water. Representative of such contactors are those disclosed in U.S. Pat. Nos. 3,827,559; 3,894,953; 3,904,525; 3,997,443; 4,083,746; and 4,115,268.
The rotors of the contactors are generally formed by placing discs side-by-side on a shaft, or by spirally wrapping a strip around a shaft as in U.S. Pat. No. 4,115,268. Various types of spacers are used to keep the discs spaced apart, as well as to maintain the spiral layers apart, and to increase the contact area. The spacers can be separate elements or they can be integrally formed in the discs or spirally wound strip.
The environment in which the rotors are used is highly corrosive. Because of this, the large size of the rotors and the large number of them needed in waste water treatment plants, it is advisable to make them of an inert, tough, and inexpensive material which can be readily shaped and assembled into the rotors. Commercially available polymeric materials such as polyethylene, polypropylene and polystyrene are representative of those which can be used.
Thissen U.S. Pat. Nos. 4,115,268 and 4,083,746 disclose vacuum thermoforming three dimensional geometrical shapes into thermoplastic polymeric sheet material to thereby form integral spacers in the sheet and the subsequent forming of disc rotors and spiral rotors therefrom. The vacuum thermoforming processes disclosed in such prior art operate on a unit basis in which one polymeric sheet is formed on a mold, removed and a new sheet then vacuum thermoformed. Such a procedure has been considered essential because of the necessity to seal the sheet to the mold border to prevent air from entering when air between the sheet and the mold is removed to lower the pressure therein so that atmospheric pressure can press the previously heated and plastic or softened polymeric material into contour contact with the mold surface. The process is similar when a long strip of polymeric sheet material is vacuum thermoformed except that consecutive areas of the strip, rather than separate sheets, are subjected to the described operation. Such processes, involving a start-stop sequence, are inherently slow, limit production, increase costs and require more labor than is desired.